Well, I'm a crocheter, and you certainly have a dilemma here! I'm not sure if you mean she wants the two afghans sewn together and then quilted, or sewn separately. But what I am picturing is - you'll need to sew border around the afghan (let's say it's lightweight flannel), then attach a flannel backing with the right side facing up (so you would see it through the spaces in the crochet). Then you'd have a matching piece of flannel for the back, with a very lightweight batting in between the two pieces of flannel. You could sew the batting to the back flannel - the crocheted afghan to the front flannel - then use the birthing method to sew them all together, turn right side out, and you've got an afghan with a cozy flannel back. My only issue with this would be - that's going to be one HEAVY afghan! Crochet by its nature tends to be more weighty than knitting, and once you add backing and batting....it might be like laying under that lead blanket at the dentist's office during x-rays!!
If she wants both afghans sewn together (like, back to back) you could do the same thing but again - too heavy for me!
It might help if you could post a photo of the afghans... I'm sure there are a few more crocheters around here... maybe we could put our heads together and help you out.